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Gemini Planet Imager Coronagraph Testbed Results Gemini Planet Imager coronagraph testbed results Anand Sivaramakrishnana,b,R´emi Soummerc, Ben R. Oppenheimera, G. Lawrence Carrd, Jacob L. Meya, Doug Brennera, Charles W. Mandevillea, Neil Zimmermana,e, Bruce A. Macintoshf, James R. Grahamg, Les Saddlemyerh, Brian Baumanf, Alexis Carlottii, Laurent Pueyoj, Peter G. Tuthillk, Christophe Dorrerl, Robin Robertsa, Alexandra Greenbaumm, a American Museum of Natural History, 79th Street at CPW, New York, NY 10024, USA b Stony Brook University, Stony Brook NY 11794, USA c Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218 USA d National Synchrotron Light Source, Brookhaven National Laboratory, Upton, NY 11973 USA e Columbia University, 550 West 120th Street, New York, NY 10027 USA f Lawrence Livermore National Laboratory, 7000 East Ave Livermore, CA 94551 USA g Astronomy Department, University of California at Berkeley, CA 94720 USA h Hertzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria BC V9E 2E7 Canada i Princeton University, Princeton NJ 08544 USA j Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena CA 91109 USA k University of Sydney School of Physics 2006 NSW Australia l Aktiwave, 241 Ashley Drive, Rochester, NY 14620 USA m Rensselaer Polytechnic Institute, 110 Eighth Street, Troy, NY 12180 USA ABSTRACT The Gemini Planet Imager (GPI) is an extreme AO coronagraphic integral field unit YJHK spectrograph destined for first light on the 8m Gemini South telescope in 2011. GPI fields a 1500 channel AO system feeding an apodized pupil Lyot coronagraph, and a nIR non-common-path slow wavefront sensor. It targets detection and characterizion of relatively young (<2GYr), self luminous planets up to 10 million times as faint as their primary star. We present the coronagraph subsystem’s in-lab performance, and describe the studies required to specify and fabricate the coronagraph. Coronagraphic pupil apodization is implemented with metallic half-tone screens on glass, and the focal plane occulters are deep reactive ion etched holes in optically polished silicon mirrors. Our JH testbed achieves H-band contrast below a million at separations above 5 resolution elements, without using an AO system. We present an overview of the coronagraphic masks and our testbed coronagraphic data. We also demonstrate the performance of an astrometric and photometric grid that enables coronagraphic astrometry relative to the primary star in every exposure, a proven technique that has yielded on-sky precision of the order of a milliarsecond. Keywords: apodized pupil Lyot coronagraph, National Synchrotron Light Source, adaptive optics, corona- graph, coronagraphic astometry, coronagraphic photometry, high-contrast imaging, integral field spectrograph, extrasolar planet 1. INTRODUCTION Astronomy is at a new frontier of comparative planetary science. Recent advances in adaptive optics (or AO, which corrects atmospheric disturbances to stellar light in real-time), combined with coronagraphy, a technique for suppressing the diffracted flood of light from a star to search its environs for planetary companions and faint protoplanetary disks, will enable the direct detection of warm young extrasolar Jupter-like planets within 50 Further author information: (Send correspondence to A.S.) A.S.: E-mail: [email protected], Tel: 1 212 313 7653 Ground-based and Airborne Instrumentation for Astronomy III, edited by Ian S. McLean, Suzanne K. Ramsay, Hideki Takami, Proc. of SPIE Vol. 7735, 773586 · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.858623 Proc. of SPIE Vol. 7735 773586-1 Downloaded From: http://spiedigitallibrary.org/ on 02/15/2013 Terms of Use: http://spiedl.org/terms parsecs of our Sun. These planets are at least 7 orders of magnitude fainter than their parent stars in the H and K spectral bandpasses (with central wavelengths of 1.65 and 2.1 microns respectively). The Gemini Planet Imager (GPI) is a near-IR ‘extreme AO’ (ExAO) coronagraphic instrument being devel- oped for the twin 8 m Gemini telescopes. With an advanced AO system operating at 2kHz, and 1500 channels of wavefront sensing and correction over an 8 m primary mirror, it is designed to provide a sufficiently flat stellar wavefront to a coronagraph to enable the requisite coronagraphic suppression demanded by these science goals. 2. INSTRUMENT OVERVIEW GPI consists of six integrated subsystems. 1. The opto-mechanical superstructure (OMSS) led by HIA mounts and connects all the subsystems and mates to the Gemini ISS. The AO optics and elements mount directly to the OMSS optics bench, while a flexure-sensitive frame holds other major subsystems. 2. The adaptive optics system (AO) led by LLNL, responsible for fast measurement of the instantaneous wave front, and for providing wave front control via deformable mirrors. The optical components and mechanisms for the AO system are provided by the OMSS. 3. The calibration unit (CAL) led by JPL is a high-accuracy infrared wave front sensor tightly integrated with the coronagraph. The CAL high-order wavefront sensor (HOWFS) is an interferometer that provides precise and accurate measurements of the time-averaged wave front at the science wavelength and coronagraph focal plane, so that persistent speckles caused by quasi-static wave front errors do not dominate the final image. The CAL low-order wavefront sensor (LOWFS) is a small Shack-Hartmann sensor that provides pointing, focus, and low-order aberration sensing to keep the target star centered on the coronagraph. It provides GPI with a unique capability to achieve and sustain systematic wavefront errors at the nanometer level. 4. The coronagraph subsystem (COR) led by AMNH uses a combination of apodized masks and focal plane stops to control diffraction and pinned speckles. 5. The science instrument (IFS) led by UCLA and University of Montreal, is an integral field spectrograph that produces the final scientific image or data cube, including simultaneous multiple wavelength channels to suppress residual speckle noise, and a dual-channel polarimetric capability. It also provides a diagnostic pupil-viewing mode and contains the final Lyot stop for the coronagraph. 6. The Top-Level Computer (TLC) produced by HIA, coordinates sequencing and communication between subsystems, and between GPI and the observatory. It also provides motion control for all the subsystems. 3. GPI CORONAGRAPH The baseline coronagraph for GPI is the Apodized Pupil Lyot Coronagraph (APLC).4 The light from the AO system is passed through a pupil plane A containing a transmissive apodizer mask that tapers the intensity of light across the pupil. The light is brought to a focus at a focal plane mask (FPM) where the central core of the PSF is removed. The off-axis light continues to the re-imaged pupil. The combination of the initial apodizer and the focal plane mask channels the coherent portion of the on-axis light outside the re-imaged pupil, where it is blocked by a Lyot stop. In the final focal plane, at the design wavelength, diffraction is almost perfectly suppressed. In the GPI architecture, as in the Lyot Project and P1640 coronagraphs, the FPM is a super polished mirror with a central hole, allowing the on-axis light to pass into the CAL system. The final Lyot stop is located inside the IFS dewar. Proc. of SPIE Vol. 7735 773586-2 Downloaded From: http://spiedigitallibrary.org/ on 02/15/2013 Terms of Use: http://spiedl.org/terms GPI Coronagraph Testbed MI ga. - and Grid (xyz) FoId' a --: Enid and VennnLon M2 -a Fold Callimatar _- I FPM Ellhsex_= Tweer Enclosura and Veronica OAF3 BauwL Support platform not shown Figure 1. A solid model showing the AMNH coronagraphic testbed, with the optical train in the inset. 4. THE AMNH NEAR-IR CORONAGRAPHIC TESTBED The near IR JH testbed at AMNH provided a proving ground for the technology required to develop GPI’s coronagraphic masks. The testbed consists of a passive, all-reflective optical train that is very similar to the GPI post-AO system optical train. The testbed is fed with a supercontinuum IR laser. A mirror with a stop on its surface is illuminated with a collimated beam on the testbed. This forms the testbed’s input pupil. This pupil is reimaged to a pupil image where a first coronagraphic mask is placed. This is an apodizer. A subsequent focal plane is created, again, with the same f/64 beam that GPI will use, on a superpolished mirror with a clean-edged hole at its center. This is the focal plane mask (FPM). The light reflected by the FPM then strikes another powered mirror to create a Lyot plane with the GPI size and geometry. After passing through a Lyot stop in this plane, the beam is focussed on to Veronica, a NICMOS camera which records J and H data. This camera is modeled after MONICA,1 although all optics after the dewar window were removed for use on the testbed. The camera is not equipped with a Y filter, and the lack of a cold stop made K band imaging impractical for thermal reasons. The testbed’s wavefront error is of the order of 30nm.19 5. CORONAGRAPH DESIGN Although the apodized pupil Lyot coronagraph (APLC)2–5 is a robust high-throughput coronagraph suitable for detecting young Jovian planets around nearby stars, it is, like most Lyot coronagraphs, inherently chromatic. It operates by carefully matching the pupil plane response of its FPM to the apodizer mask. Since the FPMs Proc. of SPIE Vol. 7735 773586-3 Downloaded From: http://spiedigitallibrary.org/ on 02/15/2013 Terms of Use: http://spiedl.org/terms effective size in units of λ/D is obviously a function of wavelength. By optimizing the Lyot stop geometry the APLC apodizer/FPM combination was optimized a specific 20% bandpass at Y, J, H and two bandpasses covering K. The chosen APLC mask set has excellent achromatic performance, but somewhat limited contrast in the region of 3-5λ/D.
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